AD Connect custom setup issue

[Stefano Colombo]

we're installing AD connect to sync two separate forest to the same tenant

AD Connect has been installed on DC on DOM-A.

The two companies are connected via VPN, no firewall restrictions are present.

On the DC on Company A we have added a secondary zone for the internal domain of Company B

when we run the AD connect wizard using the custom setup and we try to add the remote forest we get the following error

[31/03/2021 16:14:16] [INFO ] Verifying if the provided credentials are correct
[31/03/2021 16:14:16] [INFO ] Attempting to obtain a domainFQDN
[31/03/2021 16:14:16] [INFO ] Attempting to retrieve DomainFQDN object...
[31/03/2021 16:14:17] [SUCCESS] The provided credentials were correct
[31/03/2021 16:14:17] [INFO ] Attempting to obtain Domain Controllers associated with companyb.lan
[31/03/2021 16:14:17] [INFO ] Obtaining ForestFQDN
[31/03/2021 16:14:17] [INFO ] Attempting to retrieve ForestFQDN...
[31/03/2021 16:14:36] [ERROR ] Exception calling "GetForest" with "1" argument(s): "The specified forest does not exist or cannot be contacted."
[31/03/2021 16:14:36] [ERROR ] Cannot retrieve DCs associated to a forest named: companyb.lan.

If we run the get-adforest for the companyB domain controller we are able to retrieve data

Can anyone help us with it ?

[#2]

Multiple forests, multiple sync servers to one Azure AD tenant

Having more than one Azure AD Connect sync server connected to a single Azure AD tenant is not supported. The exception is the use of a staging server.

This topology differs from the one below in that multiple sync server connected to a single Azure AD tenant is not supported.

https://docs.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies

[#3]

I’m not using 2 AD Connect
The topology I’m using is
- 2 forest
- 1 tenant
- 1 ad sync

[#4]

I found the solution, if it can help in the future.
When we configured the DNS resolution for domain B using a secondary zone in the DomainA's DNS we replicated only the domain fqdn. We had also to configure the secondary zone for _msdcs.domain.com ( a conditional forwarder would have done as well )